1. Field of the Invention
The present invention relates to a synchronous rectifier control device and a forward synchronous rectifier circuit; in particular, to a synchronous rectifier control device and a forward synchronous rectifier circuit for determining continuous current mode or discontinuous current mode based on duty cycle of input signal.
2. Description of Related Art
FIG. 1 illustrates a diagram of a conventional forward circuit. The forward circuit is provided with a transformer T1, and its primary side has an input voltage VIN connected to a pre-stage circuit, a pulse width modulation controller PWM, an input filtering capacitor C1, an initiating resistor R1, an initiating capacitor C2, a current detecting resistor R2, a rectifier diode D1 and a transistor switch Q1 controlled by the pulse width modulation controller PWM. On the secondary side of transformer T1 are two output rectifier diodes D2, D3, an energy storage inductor L, an output filtering capacitor C3 and a voltage detector 10 formed by resistors R3 and R4.
In the above-mentioned forward converter circuit, when initially started, the input voltage VIN begins to charge initiating capacitor C2 through initiating resistor R1. When the potential in initiating capacitor C2 has been charged to a level high enough for initiating pulse width modulation controller PWM, pulse width modulation controller PWM will start to operate. Pulse width modulation controller PWM, based on the detecting signal for output voltage VO from voltage detector 10 and the detecting signal for the input current from current detecting resistor R2, adjusts the duty cycle of a control signal, i.e. adjusting the ratio of turning on periods and turning off periods in transistor switch Q1. When output voltage VO is below a predetermined voltage, the duty cycle of the control signal will be increased; contrarily, when output voltage VO is above the predetermined voltage, the duty cycle of the control signal will be reduced, thereby a stable output voltage VO can be output.
When transistor switch Q1 is turned on, input voltage VIN provides energy through transformer T1, stores energy to initiating capacitor C2 through rectifier diode D1, and stores energy to energy storage inductor L and output filtering capacitor C3 through rectifier diode D2. When transistor switch Q1 is turned off, initiating capacitor C2 discharges energy to enable pulse width modulation controller PWM to continue to operate, while energy storage inductor L discharges energy to output filtering capacitor C3 via the rectifier diode D3.
However, since there are forward voltage drop on rectifier diodes D2, D3 when current flows through, power loss thus appears. As a result, it is known, in prior art, that the rectifier diodes may be replaced with transistor switches, so as to reduce power loss therein.
Referring now to FIG. 2, wherein a diagram of a conventional forward synchronous rectifier circuit is shown, in which transistor switches Q2, Q3 are used to replace rectifier diodes D2, D3 in FIG. 1. A synchronous rectifier controller Con controls the turning-on and turning-off periods of transistor switches Q2, Q3 based on the secondary side voltage and deadtime setting signals S1, S2.
FIG. 3 illustrates a signal timing diagram of a conventional forward synchronous rectifier circuit operating in continuous current mode. In conjunction with FIGS. 2 and 3 for references, the voltages on two sides of transformer T1 are respectively V1, V2, and, when synchronous rectifier controller Con detects that voltage V1 in transformer T1 increases, a first synchronous signal is generated for controlling transistor switch Q2 to become conducting. At this moment, the current in transformer T1 flows from voltage V1 side to the other side of the transformer through energy storage inductor L, output filtering capacitor C3 and transistor switch Q2. Synchronous rectifier controller Con, based on dead zone setting signal S1, makes transistor switch Q2 to be cutoff in advance for a deadtime DT1 before conducting time Ton. When transistor switch Q2 is cutoff and after elapsing of deadtime DT1, synchronous rectifier controller Con generated a second synchronous signal to control transistor switch Q3 to be conducting, and now the energy stored on energy storage inductor L can output via the path formed by output filtering capacitor C3 and transistor switch Q3. Synchronous rectifier controller Con, based on deadtime setting signal S2, makes transistor switch Q3 to be cutoff in advance for a deadtime DT2 before cutoff time Toff. Deadtimes DT1, DT2 are set in order to avoid transistor switches Q2, Q3 from conducting simultaneously. Within deadtimes DT1, DT2, current of the secondary side could flow through the body diodes of transistor switches Q2, Q3.
Whereas, the aforementioned approach to achieve deadtime setting by means of cutting off the transistor switches in advance for a predetermined period may easily cause a situation of reverse current in discontinuous current mode. Referring to FIG. 4, wherein a signal timing diagram of a conventional forward synchronous rectifier circuit operating in discontinuous current mode is shown. While operating in discontinuous current mode, the energy storage inductor L may have already discharged all energy stored, before the pulse width modulation controller PWM on the primary side controls transistor switch Q1 to conduct in the next period, hence output filtering capacitor C3 would start to output energy in reverse direction to energy storage inductor L, as shown in FIG. 4, clearly indicated with area A that has less than 0 volts appears in voltage V2. The occurrence of reverse current may cause not only instability in output voltage VO, but also consume energy.